Magnetic detection system for detecting movement of an object utilizing signals derived from two orthogonal pickup coils

ABSTRACT

A pair of magnetic field sensors are disposed perpendicular to one another for sensing, at a single location, different directional components of a magnetic field. The output signals from the magnetic field sensors are vectorally combined to provide vector representations of the varying magnetic field. The direction of movement of the vector representations is then determined in order to indicate either the direction of movement of the object creating the variations in the magnetic field or the relative position of the object with respect to the sensors.

United States Patent Davis, J r. et al.

[ 5] Feb. 22, 1972 [72] Inventors: Paul D. Davis, Jr., Garland; ThomasE.

McCullough, Dallas, both of Tex.

[73] Assignee: Texas Instruments Incorporated, Dallas,

Tex.

[22] Filed: Dec. 31, 1969 [21] Appl.No.: 889,574

[52] US. Cl ..324/4I, 324/4, 324/8,

340/38, 340/258 [51] Int. Cl ..G0lr 33/00 [58] Field of Search ..324/4l,43, 8, 3, 4; 340/38 L,

[56] References Cited UNITED STATES PATENTS 2,741,853 4/1956 Anderson..324/43 X 2,749,506 6/1956 Emerson..... ..340/l97 X 3,355,707 11/1967Koerner ...,324/4l X Primary Examiner-Rudolph V. Rolinec AssistantExaminerR. J. Corcoran Attorney-James 0. Dixon, Andrew M. Hassell, ReneE.

Grossman, Melvin Sharp and Richards, Harris and Hubbard [57] ABSTRACT Apair of magnetic field sensors are disposed perpendicular to one anotherfor sensing, at a single location, different directional components of amagnetic field. The output signals from the magnetic field sensors arevectorally combined to provide vector representations of the varyingmagnetic field. The direction of movement of the vector representationsis then determined in order to indicate either the direction of movementof the object creating the variations in the magnetic field or therelative position of the object with respect to the sensors.

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sum 2 OF 3 OBJECT MOVING TO RIGHT oBJEcT MOVING TO LEFT- 1 96.5w) R"F/6.3(b) LP /;ST\Y /Q\ LR L VECTOR +L.P VECTOR 58 menu) R l l l .9. 32 22 2 l I l 1 LR+LPYVECTOR LR P VECTOR I 1/2 q PULSE [7 r I WIDTH MOD l 1//4 I30 f I34 X I32 COMPUTOR I0 [/6 d PULSE H WIDTH MOD WINVENTORS:

I THOMAS E. McCULLOUGH FIG.6

PATENTEDFEB 22 I972 SHEET 3 BF 3 a. @2725 wmm MAGNETIC DETECTION SYSTEMFOR DETECTING MOVEMENT OF AN OBJECT UTILIZING SIGNALS DERIVED FROM TWOORTHOGONAL PICKUP COILS This invention relates to the sensing ofmagnetic fields, and more particularly to a method and system fordetermining the direction of movement or the relative position of anobject having a magnetic moment.

It is desirable for a number of applications to be able to detect thepresence and movement of metal or magnetic objects. For instance, it isoften desirable to detect the number of vehicles passing a predeterminedhighway location and to indicate the direction of movement of thevarious vehicles. Also, there are many security environments wherein itis desirable to detect the presence of weapons and the like, such as thechecking of passengers for weapons prior to boarding aircraft of othervehicles.

Several different types of magnetic-field-sensing techniques haveheretofore been developed to provide such magnetic field detection. Anexample of a single loop detector for use in vehicle proximity detectionis disclosed in the U.S. Pat. to Barringer et al. U.S. Pat. No.3,430,22I, issued Feb. 25, 1969. Another type of system utilizing asingle sensor is disclosed in the patent to Koerner U.S. Pat. No.3,355,707, issued Nov. 28, 1967. However, such systems have not beenable to indicate the direction of travel of an object passing inproximity thereto, and have often required relatively complex circuitryinvolving threshold measurement. Other systems have thus heretofore beendeveloped wherein two magnetic sensors, such as magnetometers, have beenspaced apart along a roadway in order to determine the direction of anautomobiles movement. However, such systems are undesirably expensive inthat at least two sensors and associated circuitry are required.

In accordance with the present invention, a pair of magnetic fieldsensors have their sensing axes disposed at an angle to one another forgeneration of output signals representative of the changes in a magneticfield at a single location. Circuitry is connected to the sensors forvectorally combining the output signals into vector signals. Circuitryis provided for determining the direction of movement of the vectorsignal to indicate an aspect of the physical orientation of the objectwhich causes the variations in the magnetic field.

In accordance with a more specific aspect of the invention, a pluralityof magnetic field sensors each generate output signals representative ofa different directional component of one location of a magnetic field.Circuitry is provided which is responsive to the output signals forgenerating time derivative signals. Circuitry combines the outputsignals with the time derivative signals to produce resultant signalsrepresentative of the direction, speed and magnetic moment of the objectcausing the varying magnetic field.

In accordance with yet another more specific aspect of the invention,two magnetic field sensors generate output signals representative ofperpendicular directional components of a varying magnetic field. Eachof the output signals is differentiated, and circuitry multiplies eachoutput signal by the differential of the other output signal. One of themultiplication products is then subtracted from the other multiplicationproduct to provide a resultant signal. The polarity and magnitude of theresultant signal is sensed to determine either the direction of movementof the object creating the magnetic field, or to indicate the relativeposition of the object with respect to the sensors if the direction ofmovement of the object is known.

For a more complete understanding of the present invention and forfurther objects and advantages thereof, reference may now be made to thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a diagrammatic illustration of the magnetic field surroundinga magnetic dipole, along with arrows showing the direction of the fieldat points around the dipole;

FIGS. 2a-c are diagrammatic illustrations of the passage of a magneticdipole past a pair of perpendicular magnetic field sensors right to theinvention;

FIGS. 3a-c are waveforms taken from the output of the coils shown inFIGS. 2a-c, with the left end of the moving object being designated asthe magnetic north pole;

FIGS. 4a-c are waveforms resulting from the sensing coils shown in FIGS.Za-c with the left end of the moving object being designated as themagnetic north pole;

FIG. 5 is a vector diagram illustrating the detection of a moving vectorin accordance with the present invention;

FIG. 6 is a block diagram of the preferred embodiment of the processingsystem of the invention; and

FIG. 7 is a detailed schematic diagram of the block diagram shown inFIG. 6.

Referring to FIG. 1, the magnetic field surrounding a magnetic dipole,such as created by a ferrous object, is illustrated. Field line 10 isthe I-I-gamma contour line, while line 12 is the IOH-gamma contour line.A plurality of vector arrows are disposed along circle 14 to illustratethe field direction at various points. It will be noticed that, withrespect to an individual positioned at the center of the field, a pathleading from left to right of the individual will result in movement ofthe associated vectors in a clockwise motion.

Hence, in moving from A to B along the path A-B, the various vectorsinvolved may be seen to move sequentially clockwise. When the individualthen faces the path C-D and an object moves left to right from point Dto point C, the various field vectors move sequentially in a clockwisemotion. Conversely, if the object moves from right to left of theindividual at the dipole, the field vectors rotate sequentiallycounterclockwise. This phenomenon likewise occurs with respect to afixed sensing station as a magnetic moment moves thereby withoutchanging its direction of orientation.

Referring to FIGS. 2a-c, a sensing coil 20 is disposed at right anglesto a sensing coil 22. An object 24 has a magnetic field as illustratedby the dotted lines 26. The present inven tion is directed to sensingvariations in a magnetic field caused by the movement of the object 24.The present system thus detects any object or body that is provided witha magnetic moment, such as a magnetic dipole or a higher order fieldsuch as a magnetic quadrupole and the like. Such objects will compriseparamagnetic and ferromagnetic bodies which pass through the earthsmagnetic field and thus create magnetic field variations which aresensed by the present system. Additionally, the bodies sensed by thepresent system may comprise loops having current flowing therein togenerate a magnetic field. Alternatively, the system may sense apermanent or induced magnet as it travels thereby. The invention maythus be used to detect weapons carried by passengers as they boardaircraft, and the invention may also be utilized to detect the passageof vehicles and the like.

Referring again to FIG. 2a, when the magnetic north pole of the object24 is at the right end as illustrated, the field line 28 is thusprovided. If the left end of the object 24 is designated as the northmagnetic pole, the magnetic field line 30 exists in the oppositedirection. One end of each of the coils 20 and 22 is used as a referencepolarity point, or a normal positive polarity point of the field createdby current in the coil, as indicated by the dots on the coils. The coil20 is positioned at a right angle to the path of movement of the object24, while the coil 22 is positioned parallel to the direction ofmovement of the object 24. As shown in FIG. 2a, the object 24 ispositioned at the left of the coils 20 and 22. At position 2b, theobject 24 has moved even with the coils 20 and 22, while in FIG. 2c, theobject 24 has moved to the right of the coils 20 and 22.

As the object 24 moves from left to right past the coils 20 and 22, eachof the coils detects a magnetic field varying in time. A representationof the sensed varying magnetic fields with respect to time isillustrated in FIG. 3. Referring to FIG. 3, when the right end of theobject 24 is the north magnetic pole, the waveform 3a, is generated bythe coil 20 as the object 24 moves past the stations 2a-c, When object24 is at the position shown in FIG. 2a, the field line 28 opposes thereference positive field in the coil 20 and thus a negative field, whencompared to normal or zero, is output from the coil as indicated by thenegative peak 32. When the object 24 is at the position shown in FIG.2b, the coil 20 is even with the center of object 24 and thus senses azero effect from the magnetic field of the object 24. This zero effectis indicated in FIG. 3a by the portion 34 of the output waveform fromthe coil 20. When the object 24 is positioned at the position shown inFIG. 2c, the field line 28 tends to aid the internal field set up incoil 20, and thus a positive output peak 36 is generated by the coil.

Referring now to FIG. 3b, when the object 24 is at the position shown inFIG. 2a, the field line 28 tends to aid the positive reference fieldwithin the coil 22, and thus a positive output results from the coil 22.Additionally, when the object 24 is at the positions shown in FIGS.2b-c, the field line 28 also tends to aid the internal field set upwithin coil 22 and thus a smoothly varying positive waveform 38 isgenerated from the coil 22. The waveform 38 is at its peak as the object24 passes closest to the coil 22, as shown in FIG. 2b.

Assuming that the object 24 moves from right to left past the coils 20and 22, the object would then occupy the positions shown in FIGS. -0 inreverse order. Thus, the waveform identified generally by numeral 40(FIG. 3a) will result as the output supplied by coil during this rightto left movement of the object 24. It will be noted that waveform 40 isan identical mirror image of the waveform formed by wave peaks 32, 34and 36 previously discussed. Likewise, as object 24 moves from right toleft past coil 22, the output wave 42 will be generated for the reasonspreviously discussed.

The crux of the invention is to observe the angular motion of themagnetic field vectors sensed by the present system and caused as aresult of the passage of an object. Thus, the fields created in coils 20and 22 for each of the positions shown in FIGS. 2a-c are addedvectorally as illustrated in FIG. 3c. For instance, the vector 44 isrepresentative of the vector addition of the two coil outputs when theobject is at the position shown in FIG. 2a. Likewise, vector 46 is aresult of the vector addition of the outputs of the coils when theobject is in the position of FIG. 2!), while vector 48 is the result ofthe vector addition of the outputs of the coils when the object 24 is atthe position shown in FIG. 2c. It will be noted that as the object 24moves from left to right, the vectors 24-46 and 48 sequentially rotateclockwise.

Likewise, the vector 50 is the result of the vector addition of theoutputs of the coils when the object 24 is in the position shown in FIG.2c. Vector 52 is the result of the vector addition of the outputs of thecoils when the object 24 is at the position shown in FIG. 2b, whilevector 54 is the result of the vector addition of the outputs of thecoils when the object 24 is at the position shown in FIG. 2a. It will benoted that the vectors 50, 52 and 54 sequentially rotate in acounterclockwise direction as the object 24 moves from left to rightpast the two mutually angled coils 20 and 22.

The phenomenon of the change of direction of the resultant vectors ofthe outputs of two perpendicular sensing coils in dependence upon thedirection of movement of an object forms the basis of this invention. Aswill be subsequently described, circuitry is provided to sense thedirection of the resultant vectors from the sensing coils 22 and 20 inorder to indicate the directional movement, or relative position, of theobject with respect to the sensing coils.

FIGS. 4a-c illustrate that the present system is not dependent upon aparticular orientation of the magnetic poles of the object 24. FIGS.4a-c are obtained in the same manner as that previously described, butwith the magnetic pole of the object 24 being placed at the left endthereof. Thus, the magnetic field direction is present. When the object24 is placed in the position shown in FIG. 2a, the magnetic field 30tends to aid the internal magnetic field within the coil 20 and thus apositive peak 56 results at the output thereof. When the object 24 is atthe position shown in FIG. 2b, the magnetic field 30 has no result onthe internal field within the coil 26, and thus an essentially zero waveportion 58 results at the output thereof. When the object 24 is at theposition shown in FIG. 20, the magnetic field 30 tends to oppose theinternal field set up within the coil 20, and thus a negative peak 60exists at the output of the coil 20. Movement of the object 24 fromright to left of the coil 20 results in the waveform shown generally bythe numeral 62 and is a mirror image of the waveform comprising thepeaks 56, 58 and 60.

When the object 24 is at the position shown in FIG. 2a with respect tocoil 22, field line 30 tends to oppose the internal field set up withinthe coil 22, and thus a negative output with respect to normal appearsat the output of coil 22. Additionally, when the object 24 passesthrough the positions shown in FIGS. 2b-c, a negative output appearsfrom the coil 22. Thus, a negative-going waveform 64 appears at theoutput of coil 22, with the most negative portion appearing when theobject 24 is closest thereto as shown in FIG. 2b. As the object 24 movesfrom right to left past the coil 22, a mirror image of the waveform 64appears as waveform 66.

When the output from the coils 20 and 22 are vectorally added as shownin FIG. 4c, will be noted that the resulting vectors 68, 70 and 72rotate clockwise when the object is moving from left to right.Additionally, the resulting vectors 74, 76 and 78 may be seen to movecounterclockwise as the object moves from right to left. It may thus beseen that the orientation of the magnetic pole of the object makes nodifference in the use of the invention.

It may also be similarly shown that changing of the relative polaritiesof the coils 20 and 22 does not effect the outcome of the invention. Thevectoral relationship illustrated in FIGS. 2-4 may be seen to exist whenthe pickup coils of the invention are placed at any angle to the path ofmovement of the object 24. However, for best sensitivity, it isdesirable that the plane through the coil centers be crudely orientedwith the line of movement of the object, as within about :45".

Coils 20 and 22 are preferably disposed at 90" with respect to oneanother in order to detect orthogonal components of the varying magneticfield. While in some instances it may be desirable to change the anglebetween coils 20 and 22 it has been found that simplicity of operationresults when the sensing axes of the coils are disposed perpendicular toone another.

By detecting the movement of the vectors in the manner illustrated, thepresent invention may thus be utilized to determine the direction ofmovement of an object whose general position is known. Conversely, ifthe direction of the movement of the object is known, its locationrelative to the sensing coils may be determined by sensing the directionof the vector rotation. For example, if a monitoring test system is setup near an airline ticket booth, with the sensors located about waisthigh to the passengers, an object creating a magnetic moment carriednear the sensing device may be detected as being above or below thesensor for a given observed direction of movement of the person carryingthe object. This aspect of the invention is thus useful for detectinghidden weapons and the like.

FIG. 5 illustrates the basic principle by which the present inventiondetermines the direction of movement of vectors. Assume that a vector 80has the coordinates x+iy and changes by a small amount dx+idy to thevector position 82. It may be shown that the area A swept out by thevector in making the change is equal to /z(xdy-ydx).

The sign of area dA is positive if the motion of the vector iscounterclockwise and negative if the motion of the vector is clockwise.The present invention envisions computing area dA by developing dx/dtand dy/dt, performing multiplication and subtraction to provide2dA/dt=xdy/dtydx/dt, and then integrating with respect to time toprovide an accurate indication of area dA The sign of the resultingsignal will indicate direction, and the magnitude of the signal will beproportional to the objects speed and magnetic moment. Circuitry isprovided such that if a positive threshold is exceeded by the signal,movement of the object from left to right will be indicated, while theexceeding of a negative threshold will indicate motion of the objectfrom right to left.

FIG. 6 is a block diagram of the preferred embodiment of the inventionfor determining the direction of rotation of vectors. A pair of sensingcoils 100 and 102 are disposed with their sensing axes perpendicular toone another for sensing varying magnetic fields in the manner previouslydescribed. In the preferred embodiment, coils 100 and 102 each compriseswell known saturable core or flux gate" magnetometer bridge circuits.The output of the coils 100 is fed through an amplifier 104 to adifferentiator circuit 106. The output of the amplifier 104 is also fedback through an integrating amplifier 108 and a resistor 110 to theinput of the amplifier 104. This feedback loop tends to remove theeffects of the earth's steady state magnetic field, such that onlyvariations in the sensed magnetic field are fed to the differentiatorcircuit 106. The output of the differentiator circuit 106 is fed througha pulse width modulator 112, the output of which controls the operationof multiplier gate 114. The output of the amplifier 104 is also fed vialead 116 to the input of a multiplier gate circuit 1 18.

The electrical signal output from the coils 102 are fed through anamplifier 120 to a differentiator circuit 122. Feedback is providedaround the amplifier 120 by way of an integrating amplifier 124 and aresistance 126 in order to remove the effects of the earths steady statefield. The output of the differentiator circuit 122 is fed through apulse width modulator 128, the output of which controls the gatingoperations of the multiplier gate 118. The output of the amplifier 120is also fed via lead 130 to the input of the multiplier gate 114. Theoutput of the multiplier gate 114 is fed to the positive input terminalof a differential amplifier 132, while the output of the multiplier gate118 is fed to the negative input terminal thereof. The resulting outputis fed to logic or computing circuitry 134 for detection of the polarityand magnitude of the resulting signal.

It will thus be seen that the circuit shown in FIG. 6 multiplies thedifferentiated output of each coil by the direct output of the othercoil; and subtracts the multiplication products from one another togenerate an output signal having a polarity dependent upon the directionof movement of the signal vectors and having a magnitude dependent uponthe magnetic moment of the varying magnetic field. The present inventionthus accomplishes detection of the movement and magnitude of a varyingmagnetic field at a single sensing station with great accuracy.

FIG. 7 illustrates in schematic detail a circuit constructed inaccordance with the block diagram of FIG. 6. A first magnetic fieldsensor includes coils 200 and 202 which comprise saturable coremagnetometers. These coils are DC responsive, and therefore a high rateof speed of an object having a magnetic field is not required in orderto allow detection thereof by the present circuit. Driver secondarycoils 204 and 206 are connected to drive the coils 200 and 202 in thewell known manner. The output from coils 200 and 202 are representativeof the variances in the sensed magnetic field and are fed through atransformer 208. A driver primary winding 210 is driven by thecollectors of oscillating transistors 212 and 214. The emitters oftransistors 212 and 214 are connected to a source of positive voltage.The center-tapped coil 216 is connected across the bases of thetransistors 212 and 214 to complete the oscillator circuitry.

The output of this oscillator results in the drive frequency whichcontrols the operation of the system. The outputs from the driverprimary 210 are applied through leads 211 and 213 to the bases oftransistors 218 and 220, the collectors of which are connected acrossthe secondary of the transformer 208. The output from the secondarytransformer 208 is centertapped and applied through resistor 222 to theinput of amplifier 224. The operation of transistors 218 and 220 inaccordance with the drive frequency causes the double frequency signalfrom the magnetometer to be mixed with the drive frequency. Theresulting difference signal at the drive frequency is applied toamplifier 224. The amplified square wave signal from the output of theamplifier 224 is applied through a resistor 226 and via a synchronousfilter to the input of an operational amplifier 228.

Transistors 230 and 232 are connected at the base thereof to the driverprimary coil 210 for switched operation at the drive frequency. Thisswitched operation tends to average the square wave signal to give aclean output square wave having a positive or negative amplitudeproportional to the output signals from the coils 200 and 202. Theoutput from amplifier 228 is fed via a synchronous filter to the base ofa transistor 234 connected in an emitter follower configuration.

Transistors 236 and 238 are connected at the bases thereof to the driverprimary coil 210 for operation at the switching frequency. The outputapplied to the base of the transistor 234 includes both DC and ACcomponents, as the transistor 238 grounds the output of the signalduring one-half the period of the driving frequency. The collector ofthe transistor 236 is connected through a resistance 240 and acrosscapacitor 242 to the input of an operational amplifier 244. The inputapplied to amplifier 244 is primarily a DC term which is fed backthrough the amplifier 244 in order to counteract the steady stateresponse of the coils 200 and 202 to the earths magnetic field. Thecollector of transistor 232 is connected through a resistor 246 to thesecond input of the amplifier 244. The capacitors 248 are connectedacross the amplifier 244 to provide an integrating function thereto. Theoutput of the integrating amplifier 244 is fed through resistor 245 tothe primary of the transformer 208.

The signal appearing at the emitter of the transistor 234 is fed througha capacitor 250 so that an essentially AC signal is applied to a fieldeffect transistor 252. Transistor 252 is gated by a pulse widthmodulated signal from the Y channel in the manner to be subsequentlydescribed. The output from the emitter follower transistor 234 is alsoapplied through a field effect transistor 254 which is gated on by thedrive signal from coil 210. The output from the transistor 254 isapplied through a transistor 258, such that the demodulated signalappearing at the emitter of transistor 258 may be connected to a X-Yplotter for recording.

The output of transistor 254 is fed through a capacitor 260 and aresistor 262 which comprise a differentiator circuit. As the fieldeffect transistor 254 is only turned on during one-half cycle when theinput voltage thereto is nonzero, the square wave is not differentiated,but only the outer envelope of the square wave is differentiated, andonly during the half-cycle when the input voltage is nonzero. Thisdifferentiated signal is fed to the base of a transistor 264, thecollector of which is fed through a capacitor 266 for elimination of anyDC components thereon.

The signal is applied from capacitor 266 to a pulse width modulatorcircuit comprising a transistor 268 having the base thereof connected tothe collector of the transistor 270. The emitter of transistor 268 isconnected to the collector of a transistor 272. The emitters oftransistors 270 and 272 are commonly connected and are applied to asource of negative bias voltage. The base of transistor 270 is directlyconnected to the emitter of the transistor 274. The base of transistor274 is connected to the collector of a transistor 276. the base of whichis connected through diodes 278 and associated circuitry to a source ofdrive signals 280, which may comprise the drive signals from coil 210.Transistors 274 and 276 and the associated circuitry comprise a sawtoothwave generator operated at twice the drive frequency. The resultingsawtooth wave is applied to the base of a transistor 270. The pulsewidth modulator comprises a voltage comparator which utilizes thesawtooth waveform as a reference voltage for comparison with the inputsignal fed through the capacitor 266 to the base of the transistor 272.

As the sawtooth waveform has twice the drive frequency, the time ofcomparison of the input and reference voltages varies in accordance withthe level and polarity of the input signals. In the discussion of theoperation of the modulator, the drive signal appearing on lead 211 willbe designated as having the reference phase, and the time that the drivesignal is positive will be termed the A half-cycle and when the drivesignal is negative will be termed the B half-cycle. The output of themodulator will comprise a pulse during the A half-cycle and a pulseduring the B half-cycle. In the absence of an input signal, the outputpulses from the modulator will have equal width and will form agenerally symmetrical square wave. When a square wave is input into themodulator, one of the output pulses will be shortened and one of theoutput pulses will be lengthened. If the input square wave is positiveduring the A half-cycle, the output pulse occurring during the Ahalf-cycle will be shortened while the output pulse occurring during theB half-cycle will be lengthened. Therefore, the output of the pulsewidth modulator circuit is fed from the collector of transistor 270through a capacitor 284, and via a lead 286 to gate the operation of afield effect transistor 288.

A Y-channel sensor head and driver circuitry 290 is identical to theX-channel sensor and driving circuitry previously described, with theexception that the coils of the Y-channel sensing head areperpendicularly disposed with respect to the coils 200 and 202. Theoutput from the Y-channel sensor head is fed through a Y-channelamplification system 292 which again is identical to that previouslydescribed, including the feedback loop for elimination of response tothe earths steady state magnetic field. The amplified square wave outputfrom the Y-channel is fed through the emitter follower connectedtransistor 294 and to a field effect transistor 296 which is gated by agate signal 298. The output from the gated transistor 296 is fed througha differentiation circuit comprising capacitor 300 and resistor 302, andis then applied to the base of a transistor 304. The output is also fedto the base of a transistor 306 for application to the X-Y plotter forrecording, if desired.

The collector of the transistor 304 is coupled through a capacitor 308for removal of any DC components thereon. The resulting square waveinput is fed to a pulse width modulator circuit similar to thatpreviously described and which includes transistors 310 and 312. Thebase of transistor 312 is connected to the sawtooth wave generator. Theresulting voltage comparison results in a pulse-width-modulated signalwhich is fed from the collector of transistor 312, through a capacitor314 and then routed through lead 314 to gate the field effect transistor252.

It may thus be seen that the present circuit gates the square waveoutput from the coils 200 and 202 at the field effect transistor 252 byuse of the differentiated and pulse-widthmodulated signal from theY-channel sensor 290. Similarly, the output square wave from theY-channel sensor 290 is gated at the field effect transistor 288 by thedifferentiated and pulse-width-modulated signal from the X-channelsensor coils 200 and 202.

The pulse-width-modulated signals applied to gate transistors 252 and288 control the portions of the input signal which is passed thereby. Ifthe input signal applied to the gates is zero, the gated output is zero,regardless of the pulse widths of the modulated gating signals. Assumingan input to the gate transistor, if the pulse widths of the modulatedgating signals are equal, then the gate output will be a train of equalwidth and amplitude positive and negative pulses. This thus gives zeronet effect to the following integrator output. If during the Ahalf-cycle the input signals to the gate transistors are positive, and alengthened modulated pulse is present as a gate signal, then the gatetransistor will output a positive pulse greater in area than thenegative pulse in the following B halfcycle. The difference in area ofthe two pulses is proportional to the product of the input signalamplitude and the difference in A- and B-cycle pulse lengths. This willprovide a net effect in the following integration. If during the Ahalf-cycle the input signals to the gate transistors are negative, and alengthened modulated pulse is present as a gate signal, then the gatetransistor will output a negative pulse greater in area than thefollowing positive pulse. The operation of the gating circuits foradditional cases of input and gate signals will be readily understood.

The multiplied output signals from the field effect transistor 252 arefed to one input of a differential amplifier 320, while the multipliedsignals from the field effect transistor 288 are applied to the otherinput thereof. The resulting output signal from the amplifier 320 isdependent upon the difference between the two multiplied signals, and isof a magnitude large enough to drive the amplifier 322 which includes alarge capacitance 324 connected thereacross. The output of the amplifier322 is applied to a threshold sensor wherein the polarity of the signalmay be determined by common logic circuitry.

If the resulting output signal is positive, the object creating thevariances in the sensed magnetic field is known to be traveling in oneknown angular direction as observed from the sensor. Ifthe output fromthe amplifier 322 is negative, the object is known to be moving in theopposite direction. If the output from the amplifier 322 is zero, nomovement is present. The magnitude of the output signal from theamplifier 322 is indicative of the magnitude of the magnetic moment ofthe object to thereby assist in the classification of the object.

While the present application has been described with respect to the useof saturable core magnetometers, if will be understood that inductivecoils and suitable low level sensing amplifiers could be utilizedtherefore. Additionally, conventional solid-state integrated circuitmultipliers could be utilized in place of the field effect transistorgating technique described with respect to FIG. 7.

It will be understood that the present invention will be utilized tooperate various alarms or lights if the presence of an object moving ina particular direction or having a particular magnetic moment is sensedthereby. Such alarms or indicators may thus be used for traffic controlor for security purposes.

Whereas the present invention has been described with respect tospecific embodiments thereof, it will be understood that various changesand modifications will be suggested to one skilled in the art, and it isintended to encompass such changes and modifications as fall within thescope of the appended claims.

What is claimed is:

1. A system for detection of varying magnetic fields at one location,said varying magnetic fields being caused either by movement of anobject through a fixed external field, or by movement of an objectcarrying a source of magnetic fields relative to said one location, saidsystem comprising:

a plurality of magnetic field sensors each generating output signalsrepresentative of a different directional component at said one locationof a magnetic field,

means responsive to the output signals from said sensors for generatingtime derivative signals, and

means for combining said output signals with said time derivativesignals to produce resultant signals representative of the direction,speed and magnetic moment of said object causing the varying magneticfield.

2. The system of claim 1 wherein the sign of said resultant signals isdependent upon the direction of movement of the object causing thevarying magnetic field.

3. The system of claim 1 wherein said field sensors generate outputsignals representative of perpendicular magnetic field components.

4. The system of claim 1 wherein said magnetic field sensors comprisemagnetometers including means for removing steady state magnetic fieldcomponents from the output thereof.

5. The system of claim 1, wherein said means for combining comprisescircuitry for multiplying each said output signal by the time derivativeof the other output signal.

6. The system of claim 5 and further comprising:

means for subtracting one multiplication product from the other productto generate a signal having a polarity dependent upon the direction ofmovement of the object.

7. A system for detection of varying magnetic fields at one locationcomprising:

means for generating a pair of output signals representative of twodirectional components of a varying magnetic field at said one location,

means for differentiating each of said output signals,

means for generating multiplication products by multiplying each saidoutput signal by the differential of the other output signal,

means for subtracting one multiplication product from the othermultiplication product, and

means for sensing the polarity and magnitude of the remainder of thesubtraction operation.

8. The system defined in claim 7 wherein said means for generating apair of output signals comprises a pair of coils responsive toorthogonal magnetic field components.

9. The system defined in claim 7 wherein said means for generating apair of output signals comprises a magnetometer having coils disposedperpendicular to one another.

10. The system defined in claim 7 wherein said means for multiplyingcomprises a gate controlled by said means gr differentiating.

ll. The system defined in claim 7 wherein the varying magnetic fieldresults from an object moving relative to said one location, said objecteither carrying a source of magnetic fields or passing through a fixedexternal field, and the polarity of said remainder of the subtractionoperation is dependent upon the direction of travel of said objectcausing the varying magnetic field.

12. A method for detecting a varying magnetic field at one locationcomprising:

generating output signals representative of two directional componentsof a varying magnetic field at said one location,

differentiating each of said output signals,

generating a pair of multiplication products by multiplying each saidoutput signal by the differential of the other output signal,

subtracting said multiplication products from one another,

and

detecting the polarity and magnitude of the remainder of thesubtraction.

13. The method of claim 12 wherein said two directional components areperpendicular.

14. The method of claim 12 wherein the varying magnetic field resultsfrom an object moving relative to said one location, said object eithercarrying a source of magnetic fields or passing through a fixed externalfield, and the polarity of the remainder of the subtraction is dependentupon the direction of travel of said object causing the varying magneticfield.

15. The method of claim 12 and further comprising:

eliminating from said output signals steady state portions resultingfrom the earths magnetic field.

1. A system for detection of varying magnetic fields at one location,said varying magnetic fields being caused either by movement of anobject through a fixed external field, or by movement of an objectcarrying a source of magnetic fields relative to said one location, saidsystem comprising: a plurality of magnetic field sensors each generatingoutput signals representative of a different directional component atsaid one location of a magnetic field, means responsive to the outputsignals from said sensors for generating time derivative signals, andmeans for combining said output signals with said time derivativesignals to produce resultant signals representative of the direction,speed and magnetic moment of said object causing the varying magneticfield.
 2. The system of claim 1 wherein the sign of said resultantsignals is dependent upon the direction of movement of the objectcausing the varying magnetic field.
 3. The system of claim 1 whereinsaid field sensors generate output signals representative ofperpendicular magnetic field components.
 4. The system of claim 1wherein said magnetic field sensors comprise magnetometers includingmeans for removing steady state magnetic field components from theoutput thereof.
 5. The system of claim 1, wherein said means forcombining comprises circuitry for multiplying each said output signal bythe time derivative of the other output signal.
 6. The system of claim 5and further comprising: means for subtracting one multiplication productfrom the other product to generate a signal having a polarity dependentupon the direction of movement of the object.
 7. A system for detectionof varying magnetic fields at one location comprising: means fOrgenerating a pair of output signals representative of two directionalcomponents of a varying magnetic field at said one location, means fordifferentiating each of said output signals, means for generatingmultiplication products by multiplying each said output signal by thedifferential of the other output signal, means for subtracting onemultiplication product from the other multiplication product, and meansfor sensing the polarity and magnitude of the remainder of thesubtraction operation.
 8. The system defined in claim 7 wherein saidmeans for generating a pair of output signals comprises a pair of coilsresponsive to orthogonal magnetic field components.
 9. The systemdefined in claim 7 wherein said means for generating a pair of outputsignals comprises a magnetometer having coils disposed perpendicular toone another.
 10. The system defined in claim 7 wherein said means formultiplying comprises a gate controlled by said means fordifferentiating.
 11. The system defined in claim 7 wherein the varyingmagnetic field results from an object moving relative to said onelocation, said object either carrying a source of magnetic fields orpassing through a fixed external field, and the polarity of saidremainder of the subtraction operation is dependent upon the directionof travel of said object causing the varying magnetic field.
 12. Amethod for detecting a varying magnetic field at one locationcomprising: generating output signals representative of two directionalcomponents of a varying magnetic field at said one location,differentiating each of said output signals, generating a pair ofmultiplication products by multiplying each said output signal by thedifferential of the other output signal, subtracting said multiplicationproducts from one another, and detecting the polarity and magnitude ofthe remainder of the subtraction.
 13. The method of claim 12 whereinsaid two directional components are perpendicular.
 14. The method ofclaim 12 wherein the varying magnetic field results from an objectmoving relative to said one location, said object either carrying asource of magnetic fields or passing through a fixed external field, andthe polarity of the remainder of the subtraction is dependent upon thedirection of travel of said object causing the varying magnetic field.15. The method of claim 12 and further comprising: eliminating from saidoutput signals steady state portions resulting from the earth''smagnetic field.